This invention relates to a process for selectively scrubbing out sulfur compounds, especially hydrogen sulfide and carbonyl sulfide, from moist gaseous mixtures by scrubbing at temperatures of below 0.degree. C., with aromatic compounds as the scrubbing liquid, wherein the moist gaseous mixture is brought into contact with liquid methanol before being cooled to the scrubbing temperature, the scrubbing liquid after absorption of the sulfur compounds is subjected to thermal regeneration, and the resultant regenerated scrubbing liquid is thereafter recycled into the scrubbing stage.
German Pat. No. 1,231,222, incorporated by reference herein, describes a process for the preparation of ammonia synthesis gas. In this method, a raw gas is first produced by the gasification of solid or liquid fuels with oxygen and steam; this raw gas, after removal of the sulfur compounds, is introduced into a stage wherein the carbon monoxide is catalytically converted into carbon fioxide, and is then subjected to a scrubbing step to remove the carbon dioxide.
The removal of sulfur compounds, especially hydrogen sulfide as well as carbonyl sulfide, is conducted by low-temperature scrubbing with xylene. The resultant xylene, loaded with sulfur compounds, after preliminary heating, is introduced into a regenerating column heated at the sump with superheated steam to about 165.degree. C., and in the column, the xylene is therein freed of the absorbed sulfur compounds.
Usually the raw gas is saturated with water vapor, at inlet temperature, methanol is introduced into this raw gas prior to cooling to the scrubbing temperature, in order to prevent ice formation. The resultant methanol, at least partly saturated with water, is withdrawn from the heat exchanger serving for the cooling of the raw gas to the scrubbing temperature and is fed into a methanol-water separating column. The regenerated methanol is recycled to the aforementioned heat exchanger. A certain residual proportion of aqueous methanol, however, is still contained in the cooled raw gas to be fed to the xylene scrubbing stage. This aqueous methanol passes into the regenerating column together with the xylene loaded with sulfur compounds during the scrubbing procedure. The sump liquid, consisting primarily of xylene and minor proportions of the methanol-water mixture, which has collected in the sump of the regenerating column, must be heated to about 165.degree. C. to drive out the sulfur compounds to the desired extent.
The conventional process aims at preventing the methanol content in the xylene from rising above 2% by weight. For this reason, a portion of the xylene withdrawn from the sump of the regenerating column and regenerated with respect to the sulfur compounds, is fed, for the purpose of methanol separation, to a distillation column while the other portion of the regenerated xylene is reintroduced into the scrubbing column. In the distillation column heated at the sump with superheated steam to about 150.degree. C., the distillation results in a liquid xylene phase and a vapor phase of methanol-water, the latter being withdrawn overhead. This methanol-water mixture is subsequently likewise fed into said methanol-water separating column.
While an acceptable process, there is, nevertheless, room for improvement in the above-described conventional system. Specifically, the part of the plant serving for raw gas purification comprises a relatively large number of separating columns, each requiring operating energy. In particular, a comparatively large quantity of high-pressure steam is required to drive out the absorbed components from the scrubbing medium and to keep the methanol content of the scrubbing medium at a low level, i.e. below 2% by weight.